A liquid crystal display (referred to as LCD, hereinafter) is composed of display picture elements disposed in a matrix arrangement and driving circuits which drives the display picture elements. In order to improve the displaying performance of the LCD, high electric field effect mobility is required for the transistors constituting the driving circuits, since a higher operation speed is demanded for such transistors compared with transistors constituting display picture elements.
Recently, fairly high electric field effect mobility has been achieved by using a polycrystalline silicon film as an active layer of a thin film transistor (referred to as TFT, hereinafter) constituting driving circuits.
Hence, TFTs (polycrystalline silicon TFTs) comprising polycrystalline silicon films as active layers have come to be used not only for transistors constituting display picture elements but also for transistors constituting driving circuits. Also an integrated type of LCD, where both of display picture elements and driving circuits are composed of polycrystalline silicon TFTs and formed on the same insulating substrate, has been developed.
For the LCD in which the display picture elements and the driving circuits are composed of polycrystalline silicon TFTs, higher mobility (higher speed) of the polycrystalline silicon TFT is required as the display picture element becomes finer.
A polycrystalline silicon film comprises multiple monocrystal grains and crystal grain boundaries are formed between these crystal grains. The polycrystalline silicon film also contains crystal defects. Therefore, when a polycrystalline silicon film is used as an active layer, electrons which pass through the polycrystalline silicon film are scattered by crystal grain boundaries and crystal defects, and cannot smoothly pass through a channel regions formed in the active layer. As a result, there is a problem that the electric field effect mobility of the polycrystalline silicon TFT cannot be so high as that of a TFT in which a monocrystalline silicon film is used as an active layer.
Therefore, there is a problem that the improvement of displaying performance of the LCD is limited when the display picture elements and the driving circuits of the LCD are composed of polycrystalline silicon TFTs.